Thermal flowmeters operate on the well-known thermal principal by which one element is heated and maintained warmer than a second element; the amount of heat required to maintain the temperature difference is a measure of the fluid mass velocity. The two elements are normally built into a single probe. When the flowmeter is to be installed into a pressurized pipe, the probe is inserted through a valve into the pipe. The probe has a relatively large diameter and so a large hole, typically ⅝ inch in diameter or larger, must be drilled in the pipe. The force exerted by the compressed air on a drill entering the pipe is sufficient to require specialized equipment to restrain the drill.